The present invention relates to direct current (DC) brushless electric motors, and particularly to the DC brushless electric motor which can be used as a cooling fan driving motor that can be manufactured with a simplified structure, inexpensive cost and prolonged life of use.
As a cooling fan driving motor, conventionally an induction electric motor was generally employed. The reason why was that the induction motor is characterized in its simplified structure and inexpensive cost to manufacture and that it has a longer life of use and no parts are subjected to tear and wear. The induction motor, however, had a drawback that it can only be used where an alternate current source is available on one hand, while on the other in the course of miniaturizing electronic equipment and promoting popularity of its use in diversified areas there came up a requirement that all the power sources in the electronic equipment should be unified into low voltage direct current, and, in order to cope with this requirement, to operate the cooling fan driving motor with low voltage direct current has become necessary.
To satisfy this requirement, the use of a usual DC electric motor equipped with commutators and brushes accompanies such inconveniencies as contamination of the electronic devices by the wear dust that are generated from brushes and a shorter life of use as compaired with induction motors, so that the employment of a DC brushless electric motor with no brushes is now proposed.
A conventional example of DC brushless electric motor is explained hereunder along with FIG. 1. FIG. 1 (a) is a cross sectional view of its main structure, while FIG. 1 (b) shows the current control circuit which controls the flow of current to the stator windings. In FIG. 1, 1 represents the rotor axle, 2 is the rotor hub, and 3 is the permanent magnet. These 1, 2 and 3 consist the rotor of the motor. A semicircle of the cylindrical permanent magnet is magnetized as N pole, while the remaining semicircle is magnetized as S pole. 5-1 through 5-4 are the pole shoes of the stator's iron core, 6-1 through 6-4 are stator magnetic poles, 7-1 through 7-4 are the stator windings that are wound around the stator magnetic poles 6-1 through 6-4 respectively, 8 is the stator yoke, 9-1 and 9-2 are the magnetism detectors that detect the magnetic position of the permanent magnet 3, Q-1 through Q-4 are the transistors that control the current which flows through the respective stator windings 7-1 through 7-4 , and R-1 through R-8 are resistors respectively. 4 is the gap between the outer peripheral surface of the permanent magnet and the inner peripherial surface of the pole shoes 5-1 and 5-4 of the stator iron core. The magnetism detectors 9-1 and 9-2, in the example shown in FIG. 1, are firmly mounted respectively on the line passing through the respective centers of the stator magnetic poles 6-1 and 6-2.
This DC brushless electric motor is operated in the following manner. When the power source supply terminals .sym. and .crclbar. are charged with the designed DC voltage, the magnetism detector 9-1 of FIG. 1 (a), in the status as shown in the figure, does not put out any output since it opposes the neutral point (pole shifting point of N pole between S pole) of the permanent magnet 3, so that no current flows through the bases of the transistors Q-2 and Q-4 and also no current flows through the stator windings 7-2 and 7-4, whereas the magnetism detector 9-2, as it opposes the S pole of the permanent magnet 3, detects this S pole to give a .sym. voltage to the base of the transistor Q-1 and .crclbar. voltage to the base of the transistor Q-3. This turns the transistor Q-1 conductive and flows the current through the stator windings 7-1. The magnetic field formed by this current generates a counterclockwise revolution force at the S pole of the permanent magnet and the rotor starts to revolve to the counterclockwise direction. Assuming the status wherein the rotor revolved slightly counterclockwise, the permanent magnet opposing the magnetism detector 9-1 assumes S pole, whereat the transistor 9-1 detects the magnet field of S pole and puts out a .sym. voltage to the base of the transistor Q-2 and a .crclbar. voltage to the base of the transistor Q-4, whereby the transistor Q-2 becomes conductive to flow the current through the windings 7-2 and generates the revolving force for revolving the rotor towards the counterclockwise direction which rotates the rotor in that direction jointly with the windings 7-1. The revolution force generated by this current that flows through the winding 7-1 continues to force the rotor to revolve until it rotates 90 degree counterclockwise from the position shown in the drawing and when it passes rotating 90 degree the permanent magnet that opposes the magnetism detector 9-2 turns into N pole, whereat the output of the magnetism detector gives a .crclbar. voltage to the base of the transistor Q-1 and a .sym. voltage to the base of the transistor Q-2, whereby the transistor Q-1 becomes non-conductive while the transistor Q-3 becomes conductive to permit the current flow through the winding 7-3. This generates the revolving force that rotates the rotor in the counterclockwise direction and rotates the rotor jointly with the winding 7-2. Thereafter, it operates likewise always to flow the current through the two windings neighboring each other, and it generates the revolution force that rotates the rotor continuously in the designed direction (counterclockwise direction in the above cited example) by switching the conductivity from the winding that was conductive to the non-conductive winding which is located in the direction counter to the rotation. According to this structure, wherever the rotor positions it generates revolving force immediately upon applying the current, and as there is no position of rotor that reduces the revolution force to nil it exhibits excellent characteristics with which it finds its way into diversified usage.
The brushless electric motor as shown in FIG. 1, however, requires such many related circuit components and complicated assembly processes as four stator windings, four transistors that control the current flow of each winding, two magnetism detectors, resistors R-1 through R-8 and so forth and its cost is expensive, so that conventionally its use has been only limited to rather sophisticated purposes like metrological instruments and information processing systems. Accordingly, to use a brushless electric motor which has the structure as shown in FIG. 1 for driving the cooling fan that was so far using those inexpensive induction motors will sufficiently satisfy the purpose capacitywise, but it is not appropriate in the point of cost.
The present invention is to provide an inexpensive brushless electric motor with a simplified structure which suits such specific usage as for driving cooling fan by solving the above-mentioned problematic points. First, let's compare and study the required features for the motor driving cooling fan and the characteristics of the brushless electric motor shown in FIG. 1. The revolving power characteristics required for the cooling fan is that in view the character of the fan driving power which almost proportionates to the square of the fan's revolving velocity, the start-up revolution power which is required for start-up will be sufficed by a minimal revolution power that will barely overcome the frictional revolution power, and the most desirable feature is that the maximum revolution power should be attained at around the related velocity with the generated revolution power being increased in the course of acceleration after start-up, so that the characteristics of the conventional induction electric motor could be said most fitted for a fan driving electric motor. As against this, the brushless electric motor has a declining characteristic with which a decrease in the current flow reduces the revolution power as the rotation is accelerated while it generates its maximum revolution power at the time of start-up under a designed voltage supplied. In order to generate at its high speed revolution the revolution power that is required by the fan, therefore, the motor which drives the fan should be the one that is capable of generating far greater revolution power at the start-up. The present invention is, therefore, to provide a DC brushless electric motor with a lower cost and simple structure, which fits for driving a fan by reducing the number of components which result excessive features of the brushless electric motor with the structure as shown in FIG. 1 for fan driving to arriving at the equilibrium point of its performance and cost.
First, contemplating to halve the number of structural components, as per FIG. 1, which are employed in plurality, the stator magnetic poles and the stator windings which count four respectively can be halved to two respectively, also the magnetism detectors can be reduced from two to one, and the current controlling transistors can be reduced from four to two, as shown in FIG. 2. Including resistors, the circuit components can be reduced to a half whereby it simplifies the structure and it appears as if a wide reduction in cost is possible, but the structure shown in FIG. 2 involves a major drawback. In FIG. 2 (a), the rotor is positioned on the outside and the stator is placed on the inside, whereby reversing the respective positions from FIG. 1, but this does not constitute any problem in substance. 3 is magnetized as N pole and S pole and is a permanent magnet of a hollow cylindrical shape supported in a manner free to rotate; 8 is the stator yoke; 6a and 6b are the stator magnetic pole; 7a and 7b are the stator windings; 5a and 5b are the pole shoes provided at the respective stator magnetic poles; 9 is the magnetism detector; 4 is the gap between the outer periphery of 5a and 5b and the inner periphery of the permanent magnet 3, 10 is the gap in the peripheral direction between the end-points of pole shoes 5a and 5b respectively; 31 and 32 are the imaginary neutral lines respectively at where the magnetic poles of the permanent magnet 3 reverse their magnetism. In the brushless electric motor of the structure as shown in FIG. 2 (a), the shift in the magnetic resistance between the permanent magnet 3 and the stator iron core when the rotor is rotated one full revolution without flowing the current through the stator windings 7a and 7b is represented in the curve (1) of FIG. 2 (c), and it becomes minimum at the position shown in FIG. 2 (a), namely where the line connection the neutral line 31 and 32 crosses perpendicularly the center line A--A' of the stator magnetic poles 6a and 6b, and it becomes maximum at the position where the permanent magnet 3 rotates by 90.degree. from the present position, while thereafter it repeats minimum and maximum alternately at each time the rotor rotates by 90.degree.. The reason why for this is that on account of the arranged shape of the pole shoes 5a and 5b the magnetic resistance within the stator iron core becomes minimum in the direction of the central line A--A' of the magnetic poles 6a and 6b, and becomes maximum at the direction perpendicular to the above.
Accordingly, if the motor is left without flowing the current through the stator windings 7a and 7b, the rotor stays stationary, by the action of the magnetic attractive force between the permanent magnet 3 and the respective pole shoes 5a and 5b, at the position where the magnetic resistance between both becomes minimum, namely the position where the pole center of the permanent magnet 3 and the center line A--A' of the magnetic poles 6a and 6b becomes identical as shown in FIG. 2 (a), and the status of the magnetic flux of the permanent magnet 3 at this position is represented by the curve (2) in FIG. 2 (c), so that, if the magnetism detector 9 is positioned at the direction perpendicular to the center line A--A' of the magnetic poles 6a and 6b, the magnetism detector 9 is positioned on the neutral line 31-32 of the permanent magnet 3 when the rotor becomes stationary, and as there is no output from the magnetism detector 9 the current control device does not operate and accordingly no current flows through the stator windings 7a and 7b even if any voltage is charged from the .sym. and .crclbar. terminals of the control circuit of FIG. 2 (b), whereby the rotor is unable to start rotation by itself from the stationary position.
In this regard, the following examples are the steps which have been conventionally employed for providing a self-starting capability to the motor with a structure as shown in FIG. 2 (a). That is to place a small piece of magnetic material on the outer peripheral surface of the pole shoes 5a and 5b at the position shifted by a certain angle from the center line of the stator magnetic poles 6a and 6b in the revolving direction of the rotor, and to shift the stationary position of the rotor toward its revolving direction by the attractive force of this piece and the permanent magnet. Another means suggested for this purpose is to have the outer peripheral shape of the pole shoes 5a and 5b reduce to non-concentric and to make the diameterwise dimension of the gap 4 wider on one side and narrower on the other side within a certain angle centering on the center line of 6a and 6b, whereby shifting the stationary position of the rotor by transforming the magnetic resistance value between the rotor and the stator.
The former format in which a small piece of magnetic material is installed, however, has an inconveniency that it produces vibration noise by the working of the alternating magnetic field of the permanent magnet, and the latter structure modifying the gap dimension has an inconveniency that it does not provide any answer to the cost saving as it degrades its mass-productivity on account of the complicated outer periphery shape of the pole shoes.